InP-based high electron mobility transistors (HEMTs) have been used, which can operate in the millimeter-wave (about 30 to about 300 GHz) or submillimeter-wave (about 300 GHz to about 3 THz) regions, for example.
In InP-based HEMTs, In0.53Ga0.47As which lattice-matches with an InP substrate have been used as a material of a channel layer (electron transit layer). Further, pseudo lattice-matched channel materials have been used, where the indium (In) content is increased to about 70-80% in order to reduce the effective mass of carrier electrons and further increase the device speed. Currently, such InP-based HEMTs can achieve a cut-off frequency fT, which is a barometer indicating device speed enhancement, of 600 GHz or higher.
Additionally, composite channel HEMTs have been used, which use, as a channel layer, an InGaAs/InAs/InGaAs quantum well layer which includes InAs having an even further reduced energy (potential) of the conduction band in InGaAs. In such HEMTs, since the effective mass of electrons in the ┌ valley in the InAs layer is smaller than the effective mass of electrons in the ┌ valley in the InGaAs layer, many electrons as possible exist in the InAs layer, thereby increasing the velocity of electrons in the channel layer. This can achieve a further speed enhancement. In such InGaAs/InAs/InGaAs composite channel HEMTs, not only the cut-off frequency fT but also the maximum oscillation frequency fmax exceed 600 GHz.